1. Field
The present invention relates to disk drives for computer systems. In particular, the present invention relates to a disk drive compensating for rotational vibrations, physical shocks, and thermal popping.
2. Description of the Related Art
FIG. 1 shows a prior art disk drive comprising a disk 2 and a head 4 connected to a distal end of an actuator arm 6 which is rotated about a pivot by a voice coil motor (VCM) 8 to position the head 4 radially over the disk 2. The disk 2 comprises a number of concentric data tracks 10 each partitioned into a number of data sectors. Access operations are performed by seeking the head 4 to a target data track, and performing a write/read operation on the data sectors within the data track. The disk 2 comprises embedded servo sectors 120-12N having position information recorded therein, such as coarse position information (e.g., a track address) and fine position information (e.g., servo bursts). Control circuitry 14 processes the read signal 16 emanating from the head 4 in order to demodulate the servo sectors 120-12N into a control signal 18 applied to the VCM 8 in order to position the head 4 over the target data track.
The control circuitry 14 in FIG. 1 compensates for rotational vibrations, which can degrade the servo performance, as well as physical shocks to the disk drive which may cause the head 4 to deviate from the target track and corrupt data recorded in an adjacent track during a write operation. For rotational vibrations, the control circuitry 14 employs a technique referred to as Rotational Acceleration Feed-Forward (RAFF) compensation wherein a feed-forward signal is generated so that the servo system follows the disturbance in the servo loop due to a rotational vibration which is detected by a suitable accelerometer, such as a pair of piezoelectric sensors 20A and 20B shown in FIG. 1. The output of the vibration sensors 20A and 20B are amplified/filtered using suitable circuits 22A and 22B, the outputs of which are subtracted and summed, and then further amplified/filtered using suitable circuits 24A and 24B to generate a RAFF difference signal 26 and a RAFF sum signal 28. The control circuitry 14 processes the RAFF difference signal 26 and the RAFF sum signal 28 to generate a suitable feed-forward compensation signal injected into the servo control loop.
A second accelerometer, such as a piezoelectric sensor 30, is used to detect physical shocks to the disk drive. Typically the piezoelectric sensor 30 would be responsive to physical shocks in the 0.1 to 1 kHz range. The output of the piezoelectric sensor 30 is amplified/filtered using a suitable circuit 32 to generate a shock detect signal 34 monitored by the control circuitry 14. The circuit 32 is typically a filter that enhances signals in the same range as the physical shocks, i.e., 0.1 to 1 kHz. If during a write operation the magnitude of the shock detect signal 34 exceeds a threshold (positive or negative), the control circuitry aborts the write operation to help minimize adjacent track overwrite.
Thermal popping is another disturbance to the servo system due to mechanically mated parts that have different coefficients of thermal expansion. Similar to physical shocks, thermal popping may cause the head 4 to deviate from the target track and overwrite an adjacent track during a write operation. However, thermal popping typically manifests at much higher frequencies than physical shocks, and if the bandwidth of the physical shock detector shown in FIG. 1 is increased to cover thermal popping events, the shock detector may detect too many false shock events which will degrade the performance of the disk drive.
There is, therefore, a need to reliably detect and compensate for rotational vibrations, physical shocks, and thermal popping in a disk drive.